1. Field of the Invention
The present invention relates to foamed plastics which can be molded economically.
2. Discussion of the Prior Art
The conventional foamed plastics have been produced by the following three methods.
The first method is based on chemical foaming, in which a blowing agent is mixed into a resin raw material and the foamed plastics are formed by melt-extrusion. This method is disadvantageous in that the temperature should be controlled uniformly and the extent of foaming and the cell size vary greatly depending on the amount of time which has passed after mixing of the blowing agent. The second method is based on gas blowing, in which the resin raw material or the nucleating agent mixed into the resin raw material is impregnated with a gas and the resin is formed by melt extrusion. This method is disadvantageous in that the pressure should be reduced to a certain value during extrusion molding and it is very difficult to keep constant the extent of foaming. The third method is based on gas injection, in which a gas at a certain pressure is injected into the extruder. This method provides foamed plastics having a uniform extent of foaming, but is disadvantageous in that the extruder requires a special structure and expensive auxiliary equipment is required to keep the gas at a constant pressure. Thus, the present invention is intended to provide foamed plastics which can be molded economically without the aforesaid disadvantages of conventional methods.